Organic semiconductor devices require milder film-forming conditions than conventional inorganic semiconductor devices. Because it is possible to form semiconductor thin films on various substrates and at ordinary temperature, there are expectations for low cost and flexible nature by forming thin films on polymer films and the like.
Organic semiconductor materials thus far studied include polyphenylenevinylene, polypyrrole, polythiophene, oligothiophene, as well as polyacenes such as anthracene, tetracene and pentacene. It has been reported that polyacenes, in particular, have high crystallinity due to their strong intermolecular cohesive force, resulting in high carrier mobility and resultant superior semiconductor device characteristics.
The form of polyacene applied to a device includes a vapor deposition film or single crystals, and its application to transistors, solar cells and lasers has been studied (Shone et al., Science, 289, p559 (2000); Shone et al., Science, 287, p1022 (2000); Dimitrakopourasu et al., Journal of Applied Physics, 80, p2501 (1996); Shone et al., Nature, 403, p408 (2000); and Croke et al., IEEE Transaction On Electron Devices, 46, p1258 (1999)).
However, because these vapor deposition films and single crystals are formed in a vacuum vessel, they require expensive and complex equipment and in the case of single crystals, useful devices have been limited in size.
On the other hand, a method for forming a thin film of pentacene that is a kind of polyacene by applying a solution of a pentacene precursor on a substrate and heating it has been reported (Brown et al., Journal of Applied Physics, 79, p2136, (1996)). Since polyacenes are hardly soluble, a soluble precursor was used to form a thin film, which was heated to convert it into a polyacene.
However, the method using a precursor required treatment at a temperature as high as 150° C. to convert the precursor to a polyacene. In addition, non-reacted portions remained because a complete conversion to the polyacene was difficult, and denaturation due to high temperature occurred.
Polyacenes having substituents have been reported by Takahashi et al. (Journal of American Chemical Society, 122, p12876 (2000)), Graham et al. (Journal of Organic Chemistry, 60, p5770 (1995)), Anthony et al. (Organic Letters, 2, p85 (2000)) and Miller et al. (Organic Letters, 2, p3979 (2000)). These reports describe derivatives of various polyacenes having introduced substituents, but no description has been given as to their characteristics as organic semiconductor materials nor a method for forming thin films.
Hence, it is an object of the present invention to solve the problems of prior art described above and provide a solution for organic semiconductor that can be used to easily form a defect-free organic semiconductor thin film with high crystallinity at a low cost. Another object of the present invention is to provide a defect-free organic semiconductor thin film and a method for forming the same. Yet another object of the present invention is to provide an organic semiconductor device with superior electronic characteristics.